if you really want to try linux and want to understand more and learn as you go I would recommend Gentoo. There is a ton of excelllent guides and info on thier site on getting it up and running. The install process is slow and time consuming but you'll understand more about how to use Linux and know some commands when you are done. Gentoo was the first distro of linux it tried and the experience of doing a text based was much more valuable than installing Mandrake or Fedora....
I have installed Mandrake, Red Hat, Fedora, Debian, and a few others out of boredom on a spare machine. Out of all the installs and distros Gentoo is the one that I prefer. Mandrake 10 didn't like my machine and insisted that it locked up all the time....Red Hat was installed because I was working on a class project required that specific distro...Fedora ran very very slow compared to Mandrake or Gentoo...I had no problems with Gentoo and had it on my machine and used it everyday until the hard drive died in the machine.... I installed Debian on one of the iMacs we have at work becuase I was bored and wanted something to play with.
Others have recommended the distros that are easy to start out with but if you really want challenge my suggestion is try Gentoo
One of the benefits these "commercial" distributions charge for is support, rather than the product itself. Usually (but not always), you can get a free version of the same distribution (as in the case of RedHat, for example), but without the benefit of support. Most of the time, it's larger companies who end up paying for distributions because they need the support.